Conventional gas electron multipliers (GEM) have been used to detect radiation, such as charged particles, gamma rays, x-rays, neutrons and ultraviolet rays.
When the radiation, which is the detection target, enters such a gas electron multiplier, it uses electron avalanche effects to multiply photoelectrons released from gas atoms as a result of the interaction between radiation and a gas through photoelectric effects and enables to detect the radiation as an electrical signal.
FIG. 1 is a cross sectional diagram schematically showing a configuration of a radiation detector using a conventional gas electron multiplier.
The radiation detector 100 shown in FIG. 1 is composed of an outer chamber 102 filled with a predetermined gas for detection, and detector elements inside the chamber 102, which are a drift electrode 104 and a collecting electrode 106, and a first gas electron multiplication foil (GEM foil) 108 and a second gas electron multiplication foil 110 placed between the drift electrode 104 and the collecting electrode 106 at a predetermined distance TR.
Here, as the gas for detection to be filled in the chamber 102, mixed gas of rare gas and quencher gas is generally used. For example, the rare gas may be He, Ne, Ar, Xe or the like and the quencher gas may be CO2, CH4, C2H6, CF4 or the like. In addition, the fraction of the quencher gas mixed with the rare gas is appropriate to be 5% to 30%.
Here, the chamber 102 filled with the predetermined gas for detection, the first gas electron multiplication foil 108 and the second gas electron multiplication foil 110 form a gas electron multiplier. The first gas electron multiplication foil 108 and the second gas electron multiplication foil 110, each of which is made of a plate-like multilayer body having the same configuration, are to provide a function to multiply charge using electron avalanche effects.
In further detail, the first gas electron multiplication foil 108 (the second gas electron multiplication foil 110) is composed of a plate-like insulation layer 108a (110a) made of resin having a thickness t0 of 50 μm, and flat metal layers 108b and 108c (110b and 110c) overlaid on both surfaces of the insulation layer 108a (110a). In addition, a large number of through-holes 108d, 110d are formed for condensing the electrical field in the first gas electron multiplication foil 108 and the second gas electron multiplication foil 110, respectively.
In addition, the radiation detector 100 is equipped with a power supply section 112 for applying voltage to the metal layers 108b, 108c, 110b, 110c and the drift electrode 104, and a detecting unit 114 connected to the collecting electrode 106.
In the above described configuration, a predetermined voltage is applied from the power supply section 112 to the metal layers 108b, 108c, 110b, 110c and the drift electrode 104 in the radiation detector 100 so as to generate an electric field Ed between the drift electrode 104 and the metal layer 108b, an electric field Et between the metal layer 108c and the metal layer 110b, with which the electric fields inside of the through-hole structures 108d and 110d are generated, and an electric field Ei between the metal layer 110c and the collecting electrode 106.
In this situation, the electric field Et is condensed inside the through-hole structures 108d and 110d, and electrons that have entered are accelerated to cause the electron avalanche effects. Then, the collecting electrode 106 detects the electrons multiplied through the electron avalanche effects and the detecting section 114 receives a detection signal to deduce various types of detection data.
Here, in the gas electron multiplier of the above described radiation detector 100, gas electron multiplication foils in two stages having the first gas electron multiplication foil 108 and the second gas electron multiplication foil 110 are used in order to gain a large multiplication factor of electrons due to the electron avalanche effects.
That is to say, a conventional gas electron multiplier has a structure where multiple layers of gas electron multiplication foils are used in stages in order to increase the multiplication factor of electrons.
Meanwhile, photoelectrons released when interaction between radiation and a gas occurs spread approximately several hundreds of μm.
Spread of electrons increases every time the electrons pass through a gas electron multiplication foil, and therefore, position resolution gets worse and precise position information cannot be attained, and consequently, a problem arises where an image obtained in the detecting section becomes blurred.
Detection is possible using Compton scattering or electron pair generation in addition to the photoelectric effects.
Here, the conventional art known by the present applicant at the time of the filing of the patent application is described in the above and does not relate to an invention that has been documented, and therefore, there is no conventional art information to be described.